Ungrounded power distribution systems are widely used, especially at medium voltage levels, e.g., less than 50 kV. Usually, the ungrounded power distribution systems use three phase three wire configuration. The windings of three-phase transformers and voltage regulators are either ungrounded WYE connected, or DELTA connected. The three-phase loads are DELTA connected.
The short-circuit fault analysis determines the three-phase voltages of buses and three-phase currents of branches of the distribution system when a fault, such as a short circuit, occurs within a line segment. The fault locations are conventionally modeled as independent buses, and the voltages and currents of the distribution systems are determined using numerical techniques. There are many techniques published for the analysis of short circuit faults, including symmetrical component methods, time simulation based methods, and phase frame based methods.
The typical phase frame based methods include nodal admittance/impedance matrix based method such as a method described by Chen et al., “Distribution System Short Circuit Analysis—A Rigid Approach,” IEEE Trans. on Power Systems, Vol. 7, No. 1, pp. 444-450, February 1992, and topology based methods such as a backward and forward sweep method described by Zhang et al., “A Distribution Short Circuit Analysis Approach Using Hybrid Compensation Method,” IEEE Trans on Power Systems, Vol. 10, No. 4, pp. 2053-2059, November 1995.
All those methods have limitations when applied to real time analysis of ungrounded distribution systems either in modeling accuracy, or solution efficiency.
The symmetrical component methods are designed for balanced systems, and not well suited for unbalanced systems, such as distribution systems. The time simulation methods are suitable at modeling capability and accuracy, but time consuming for practical size systems. The nodal admittance/impedance matrix based methods are suitable for modeling of most distribution systems, but have difficulties in modeling zero-impedance components, and usually take a long time to obtain solutions. The topology based methods are designed for radial distribution systems, mostly for grounded systems. The computation performance of the topology based methods is heavily impacted by the number of loops and generation sources of the distribution system.